The use of automated drilling methods is becoming increasingly common in drilling subterranean wellbores. Such methods may be employed, for example, to control the direction of drilling based on various downhole feedback measurements, such as inclination and azimuth measurements made while drilling or logging while drilling measurements.
One difficulty with automated drilling methods is that the feedback measurements are not generally made at the drill bit. It will be appreciated that there are severe space limitations very low in the bottom hole assembly (BHA) and that there are physical and operational constraints that limit how close the measurement sensors can be located to the drill bit. The sensors are therefore commonly located a significant distance above the bit such that the resulting sensor measurements are subject to a time delay related to the rate of penetration of the tool through the subterranean formation and the spatial offset between the bit and the sensors. In closed loop drilling operations, a temporal feedback delay can lead to drilling a spiraling borehole which tends to increase frictional forces between the drill string and the borehole wall. A spiraling borehole may further reduce the hole cleaning efficiency of the drilling fluid which in a worst case scenario can lead to the drill string becoming irretrievably stuck in the borehole.
Therefore there remains a need in the art for improved automated drilling methods and systems, particularly ones that can mitigate the effect of the aforementioned feedback delay and hence reduce or eliminate borehole spiraling. There is also a need for such methods and systems to compensate for drop and turn tendencies of the BHA while drilling.